1. Field of the Invention
The present invention relates to a method for the manufacturing lithographic masks using a soda glass substrate that is useful in the manufacture of integrated circuits.
2. Description of the Prior Art
There continues to be an ever dramatic increase in the complexity of integrated circuits each year. As applications develop for memories, microprocessors and minicomputers there is and increasing demand for greater complexities, higher switching speeds, and smaller devices in the integrated circuits.
As a result of these greater complexities and smaller devices, there are greater demands in the lithographic masks that must be used to make these advanced integrated circuits. This puts strains upon the existing technologies of mask making. A major example is in the making of a chromium metal and chromium oxide mask on a substrate of soda lime glass. Now that line widths are becoming very narrow and may be one micron or even submicron in width, problems are seen in the sharpness of the lines and shapes produced by the resist masking and etching of the chromium oxide and chromium metal. A phenomenon which may be termed "mouse nipping" becomes a serious problem at these narrow line widths when soda lime glass is used as the mask substrate. It is believed that the cause of this problem is due to Na.sup.+ migration from the soda lime glass substrate to the chromium oxide and resist interface. A serious adhesion problem results and the sharpness of the lines and shapes are lost.
The problem of continuity of the narrow lines and shapes can be overcome by use of more expensive quartz or other specialized glass substrates which have a purity that will not result in a migration of ions and adhesion problems. However, these types of substrates are much more expensive and have other limitations.